1. Field of the Invention
This invention relates generally to a combustion system for gas turbine gas generators, gas turbine engines, or other heat devices, which can produce significant advantages including low levels of pollutants, namely oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. Specifically, the present invention relates to single stage, controllable fuel/air ratio combustors for gas turbine engines and gas generators using fuel/air premixer assemblies with controlled variable premixer exit geometry.
2. Description of the Art
Although gas turbine devices such as engines and gas generators do not produce the majority of the nitrogen oxide emissions released into the earth's atmosphere, reducing those emissions will reduce the total and, in that regard, many countries have enacted laws limiting the amounts that may be released. The reaction of nitrogen and oxygen in the air to form nitrogen oxides, like almost all chemical reactions, proceeds faster at higher temperatures. One way to limit the amount of NOx formed is to limit the temperature of the reaction. The NOx produced in gas turbine devices is produced in the combustion process where the highest temperature in the cycle normally exists. Therefore, one way to limit the amount of NOx produced is to limit the combustion temperature.
Various attempts have been made to limit the combustion temperature and thereby NOx production in both “single stage” combustors (i.e., those having only a single combustion zone where fuel and air are introduced) and “multistage” combustors, including pilot burners where several, serial connected combustion zones having separate fuel and air introduction means are used. U.S. Pat. No. 4,994,149, U.S. Pat. No. 4,297,842, and U.S. Pat. No. 4,255,927 disclose single stage gas turbine combustors where the flow of compressed air to the combustion zone and the dilution zone of an annular combustor are controlled to decrease the concentration of NOx in the turbine exhaust gases. In the above combustors, essentially unmixed fuel and air are separately admitted to the combustor, with mixing and combustion consequently occurring within the same chamber. See also Japanese Laid-Open No. 55-45739. U.S. Pat. No. 5,069,029, U.S. Pat. No. 4,898,001, U.S. Pat. No. 4,829,764, and U.S. Pat. No. 4,766,721 disclose two stage combustors. See also German Gebrauchsmuster, 99215856.0. Again, however, fuel and air are provided to each stage at least partially unmixed with complete mixing occurring within the respective combustion zones.
Attempts also have been made to utilize separate premixer chambers to provide a premixed fuel-air flow to a combustor. Japan Laid-Open Application No. 57-41524 discloses a combustor system which appears to premix only a portion of the total fuel flow to a multistage can-type combustor in a separate mixing chamber prior to introduction to the staged combustion chambers. In U.S. Pat. No. 5,016,443, a large number of separate fuel nozzles are used to inject fuel into an annular premixer chamber. However, the complexity of the above constructions employing multiple fuel nozzles and fuel splitting devices can lead to control difficulties, as well as a high initial cost.
Single stage combustor systems using external premixers are known based on the previous work of the present inventor, such as are disclosed, e.g., in U.S. Pat. No. 5,377,483; U.S. Pat. No. 5,477,671; U.S. Pat. No. 5,481,866; U.S. Pat. No. 5,572,862; U.S. Pat. No. 5,613,357; and U.S. Pat. No. 5,638,674. These systems provide close control of the fuel/air ratio by premixing all of the fuel for combustion with essentially all the combustion air using a venturi-type mixing tube, and introducing the mixture to the combustion zone of the combustor. Significant reductions in gaseous and particulate emissions have been achieved by gas turbine engines and modules over a broad range of operating conditions, employing the inventions disclosed in the above-listed patents.
It is, however, desired to provide an improved premixer system for a single stage combustor that can reduce “flash backs” from the combustor into the premixer, which can occur when the flame speed is greater than the velocity of the fuel/air mixture in the premixer. Flash backs can adversely affect the mechanical integrity and performance of the premixer system and related structure. Specifically, it is desired to provide a premixer system that can reduce flow separation in the premixer caused by the geometrical configuration of the premixer components. Flow separation can cause flash backs into the premixer.
It is further desired to provide a premixer system that can reduce pulsations in the delivery of fuel/air mixture from the premixer into the combustion chamber. These can occur from lack of flame stability in the combustor due to excessive velocities of, as well as variations in, the mixture velocity exiting the premixer. Pulsations can adversely affect the combustor liner and engine structure.
It is further desired to provide a premixer system that can deliver fuel/air mixture into the combustion chamber in a manner that reduces the impingement of flow onto the combustor liner while maintaining a comparatively simple geometric configuration of the overall design. Impingement of the flow onto the liner wall can lead to carbon build up and decrease heat transfer performance and increase thermal fatigue.
It is further desired to provide an apparatus that is relatively less complex than other state of the art annular combustor apparatus and systems thereby facilitating ease of operation, lower initial cost and maintenance of the apparatus, and substantially improved fuel/air control by the avoidance of matching a large number of separate premixers.